1. Field of the Invention
The present invention relates to a transmit diversity method applied to a transmitting system and related transmitting system, and more particularly, to a method and a system for selecting a single transmission path from at least two transmission paths of the transmitting system in order to transmit parallel stream carried by each sub-carrier.
2. Description of the Prior Art
Orthogonal frequency division multiplexing (OFDM) technology has been widely adopted in high-speed wireless LAN network. In order to improve the transceiving quality, a transmitting system currently on the market is usually equipped with multiple transmission paths to transmit a plurality of signals simultaneously. However, use of multiple transmission paths will cause the hardware cost of the transmitting system increased, and the device itself cannot improve the transceiving quality by the way of increasing the number of the transmission paths. For this reason, a transmit diversity technology can be adopted for improving the receiving quality of the receiving end.
The simplest approach of the transmit diversity is to directly use multiple transmission paths to transmit signals without any treatment. However, since the transmitting signals in the wireless channel will arrive the receiving end via different paths, the transmitting signals will produce constructive and destructive superposition effects at the receiving end. Another simple approach is to use a cyclic shift technology, which inserts a specific delay in each OFDM time-domain signal. But this technology cannot solve the possibility of producing destructive superposition effects by multiple transmitting signals, and it only lowers the possibility of producing destructive superposition effects by multiple frequencies, which has an insignificant impact. Another possible approach is to use Space Time Block Code (STBC) OFDM coding, so that the receiving end can separate the transmitting signals from different transmission paths. But a condition that the receiving end equipped with the STBC decoding ability is required when adopting this method. In addition, a beam-forming technology is to perform a signal pre-processing operation upon the transmitting signals in order to ensure that only constructive superposition effects are produced by the transmitting signals in the receiving end. But such approach needs accurate channel estimation at the receiving end, and the signal processing will cause a large burden in the hardware of the transmitting end.